The application to general purpose amplifiers of low distortion multi-tone, low average power waveforms and high efficiency single-tone, high average power waveforms place contradictory requirements on amplifier design, i.e., multi-tone distortion is typically designed to be just good enough to meet requirements which adds excessive headroom and complexity which is of little value for single-tone waveforms and which decreases overall amplifier efficiency. Conversely, the power supply and power dissipation requirements are largely determined by the single-tone waveform, which result in being overdesigned for handling multi-tone waveforms.
As a result, amplifiers are typically larger and more expensive than they need to be so that they can handle the heat dissipation from high average power waveforms. Alternately, the specification of the amplifier may be changed in order to derate the output at high average power. For example, rather than being specified at 1 KW peak envelope power ("PEP") and average power output, the specification may read 1 kW PEP/500 W average power capability.
It is known in general purpose amplifiers to implement a compromise between low distortion of multi-tone, low average power waveforms and high efficiency of single-tone, high average power waveforms. Circuits have been devised which detect the presence or absence of a multi-tone signal and effect a stepwise reduction on the power applied to the amplifier in response thereto.
For example, the output amplifier stage of a typical prior art power amplifier is shown in FIG. 1 where the RF power from a splitter stage 2 and input transformer 4 is amplified by a FET 6 and applied through an impedance matching transformer 8 to a combiner stage 10. An external RF power control loop automatically (not shown for clarity) varies the amplitude of the input signal so that the peak envelope power output will be held at a constant value. An impedance matching transformer 8 is typically used to transform the output impedance so that the FET 6 sees a constant 12.5 ohm load impedance.
To calculate the efficiency of the power amplifier, consider the case where the input to the amplifier stage is a single-tone waveform and the output of the stage is held constant at 100 W by the power control loop. Under these circumstances, the average current draw from power supply 12 which would be necessary to supply 100 W average power output is ##EQU1##
If the supply voltage (V.sub.supply) is 50 VDC and it is sourcing 2.8 Amps RMS, then the power is (50)(2.8)=140 Watts. Since only 100 Watts is delivered to the load, the extra 40 Watts is dissipated as heat in FET 6 and the amplifier stage is only 100/140=71% efficient. To increase the efficiency of the amplifier, a reduction in the power supply voltage to 40 VDC results in power of (40)(2.8)=112 Watts. Since only 100 Watts is delivered to the load, the extra 12 Watts is dissipated as heat in FET 6 and the amplifier stage is a much improved 100/112=89% efficient. However, this improved efficiency comes at a cost of increased distortion, as described below.
Referring to FIG. 2, minimum distortion under multi-tone waveform situations requires that the FET should be operated well into the saturation region where EQU V.sub.DS &gt;&gt;V.sub.GS V.sub.t. (4)
Under ideal amplifier conditions, the FET acts as an ideal current source, with its drain current controlled by V.sub.GS as shown in FIG. 3. As V.sub.supply (and therefore V.sub.DS) is reduced and it approaches V.sub.GS -V.sub.t, the FET begins to approach the Triode region, which introduces non-linearities which appear as intermodulation distortion (IMD) in the multi-tone case and as harmonic distortion in the single-tone case. In the single tone case, the unwanted harmonics can simply be filtered off using a harmonic filter (assuming that the harmonic levels are not too severe), but in the multi-tone case, the distortion effects cannot easily be corrected.
In summary, in order to provide low distortion for multi-tone and low average power waveforms, it is advantageous to increase the supply voltage to drive the FET well into the saturation region. Providing low distortion equates to improved voice and data transmission. Alternately, to provide high efficiency for single-tone, high average power waveforms, it is advantageous to have a decreased supply voltage to minimize power dissipated in the FET. Providing high efficiency equates to reduced power consumption and reduced heat dissipation, which further results in increased FET lifespan, and decreased size and cost due to smaller heatsinks and cooling fans.
Because power amplifiers typically have several dB of gain fluctuation over their frequency range and some load impedance variation must be tolerated, lowering the power supply may prevent the power amplifier from delivering the required output power at maximum exciter output drive level. In addition, lowering the voltage of the power supply may "clip" the voltage resulting in very high levels of harmonics requiring very complex and expensive lowpass harmonic filters.
Accordingly, it is an object of the present invention to provide a novel adaptive control and method for the power supply voltage to a general purpose amplifier to optimize the operation of the amplifier for both multi-tone, low average power waveforms and single-tone, high average power waveforms.
It is another object of the present invention to provide a general purpose amplifier which provides both low distortion for multi-tone, low average power waveforms and high efficiency for single-tone, high average power waveforms.
It is yet another object of the present invention to provide a novel general purpose amplifier with decreased supply voltage requirements to minimize power dissipated in the amplifier.
It is still another object of the present invention to provide a novel general purpose amplifier which provides high efficiency which equates to reduced power consumption and reduced heat dissipation, which further results in increased amplifier lifespan and decreased size and cost due to smaller heatsinks and cooling fans.
It is a further object of the present invention to provide a novel microprocessor adaptive control of the power supply to a general purpose amplifier.
It is yet a further object of the present invention to provide a novel method of adaptively controlling in real time the power supply to a general purpose amplifier as a function of the type of input signal to the amplifier, the frequency of the input signal, and the amount by which the amplifier is overdriven.
It is still a further object of the present invention to provide a novel method of adaptively controlling the power supply to a general purpose amplifier by use of a microprocessor that will increase the power supply voltage step-wise to a predetermined level if the input signal power level to the amplifier increases or decreases a predetermined amount in a step-wise fashion.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.